Cadmium sulfide barrier layer cell



April 25, 1961 D. c. REYNOLDS CADMIUM SULFIDE BARRIER LAYER CELL Original Filed May 11, 1956 2 Sheets-Sheet l METAL METAL SEMICONDUCTOR INSULATOR CONDUCTIOIN BROAD CONDUCTION VCONDUCTION BAND CONDUCTION BAND BAND BAND FORBIDDEN FORBIDDEN FORBIDDEN ENERGY FERMI LEVEL ENERGY GAP ENERGY GAP FERMI L VEL Y QLENCE VALENCE BIND FORMED EA W VALENCE BAND BAND FROM VALENCE FORMED FROM x5 J' -N s-W5 SHELL 1 1 b 1 C sPAcE CHARGE E BARRIER A METAL WORK FUNCTION SEMI" CONDUCTOR WORK FUNCTION FERMI LEVE METALLIC o o o e o o o o /IMPURITY LEVEL CONDUCTOR N-TYPE SEMICONDUCTOR METALUC PTYPE SEMICONDUCTOR Eb EQW QF o 0 o 0 o o o O lMPURlTY LEvEL METAL WORK FUNCTION 4 SEMICONDUCTOR WORK FUNCTION h P-TYPE SEMICONDUCTOR a c 0 0 0 o o o w FERMI LEVEL IMPURITY LEVEL N-TYPE SEMICONDUCTOR it/ELL O E WM 7. W

ATTORNEYS materials.

' CADMIUM SULFIDE BARRIER LAYER CELL Donald C. Reynolds, Springfield, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force Original application May 11, 1956, Ser. No. 584,483,

now Patent No. 2,844,640, dated July22, '1958. Divided and this application Feb. 25, 1958, Ser.'No; 717,534 a p Y I "'jjci imsl cl. 136-89), v

(Granted under-Title 35, US. Code 1952), sec 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This invention relates to a cadmium sulfide barrier layer cell excitable by photons and other types of ionizing radiation and is a division of my copending application, Serial Number 584,403, filed May 11, 1956 now Patent No. 2,844,640.

As a background for insuring a sufiicient understanding of the presentinvention as claimed, solid materials are made up of atoms. All atoms have electron shells. Each electron in the various shells of an atom has a discrete electrical energy relative to the atomic nucleus. When a large group of like atoms are brought together in a solid, the electron shells of the atoms form an energy band structure. The electrical properties ofpure or ideal materials are determined by the relationship of the bands formed from the outermost shell.

Illustrative diagrams are shown in Figs. 1 and 2 of the accompanying drawings wherein Fig. 1 illustrates nited States Patent four types of band structures possible among pure or ideal materials and Fig. 2 represents potential energy curves applicable to the theory of operation of the present invention.

If the band designated valence band in Fig. la, formed from the valence shell of an atom does not have its full quota of electrons, a material of the type shown in. Fig. lais obtained. If this band is full of electrons, but the next band overlaps it, one obtains a material of the type shown in Fig. 1b, wherein no forbidden energy gap appears. These Fig. 1a and Fig. lb materials are metallic conductors with the Fermi level at the top of the filled level. However, 'if the band formed in the outermost shell of an atom is full and the next allowed band does not overlap, a forbidden energy region or gap exists between the full and the empty bands, as indicated in Figs. 10 or 1d of the drawings These materials are either semiconductors Fig. 1c or insulators Fig. 1d, depending on how large the forbidden gap is.

The invention herein directly concerns semiconductors represented in Fig. 1c of the drawings. The forbidden gap in these semiconductor materials is small enough so that at room temperature there will be some electrons excited to the conduction band. An electric field appliedto such a material will cause some conduction. These materials are intrinsic semiconductors and "the conductivity consists of the combined movements of electrons and of positive holes. Q It is usually possible to obtain solid solutions of impurities and disorders in the lattice structures of these The energy levels of the impurities usually fall in the forbidden region or forbidden energy gap.

Electronegative impurity atoms donate electrons and generally have an electronic energy level lyingclose to ice the empty band or conduction band. Electropositive impurity atoms accept electrons and generally have electronic energy levels lying close to the full band of the matrix material. These are extrinsic semiconductors and will be either N-type or P-type depending on whether they have electronegative or electropositive impurity atoms respectively incorporated in their crystal lattices. The Fermi level in these materials will in general be between the impurity level and the conduction band in an N-type material and between the impurityleveland the valence band in a P-type material. When a metallic conductor and a semiconductor, or when two semiconductors are brought into contact, a rectifyingbarn'er, such as is shown in Fig. 2, will be formed if certain conditionsare met: (a) when a metal and an N-type semiconductor are brought into contact, if the work function of the metal is greater than that of the semiconductor; (b) when a metal and a P-type semi conductor are brought into contact, if the work function of the metal is less than that of the semiconductor; and (0) when a P-type semiconductor is brought into con tact with an N-type semiconductor. In Fig. 2 energy is along the ordinate and distance is along the abscissa. The term work function is defined at page of An Introduction to Semiconductors by W. C. Dunlap, Jr., published in 1957 by John Wiley & Sons, Inc., New York City, New York, as the energy required to take an electron from the Fermi level out through the surface to infinity. Introduction to Solid State Physics by Charles Kittel, published in 1954 by John Wiley & Sons. Inc. on page 236 defines work function as the work necessary to remove to infinity an electron from the lowest free electron state in the metal. A second edition of the Kittel Work was published in 1956 wherein the definition was repeated on page 266 and developed further at page 387 and elsewhere. The semiconductor here of interest .is cadmium sulfide and hence the term Work function is used herein as being with respect to cadmium sulfide.

When hole-electron pairs are created in the barrier region, or. if they can diffuse to the barrier region, they will be in the influence of the barrier layer field with the electron being drawn to the positive space charge and the hole being drawn to the negative space charge, in which situation electrical energy may be drawn away from the cell.

When these hole-electron pairs are created by light it is called a photovoltaic cell. They may also be cre ated by various types of nuclear radiation, such as electrons, gamma rays, X-rays, etc.

In most cells the photons creating the hole-electron pairs must have enough energy to excite an electron across the forbidden energy gap. In cadmium sulfide cells this is not necessary, the photovoltaic effect can be obtained from impurity levels as well as the intrinsic level, where the electron is excited across the gap.

A representative publication which elaborates further on the above theory is Electrons and Holes in Semiconductors, written by William Shockleyand published in 1955 by the D. Van Nostrand Company, Inc. of New York City, New York.

A general statement of the nature of the present invention as claimed is the conversion of radiation energy into electrical energy. The substance of the present invention is the provision of a photovoltaic cell which converts radiation energy into electricity.

A. general object of the present invention as claimed is to provide a means and a method for converting energy from the sun into electrical energy.

Cadmium sulfide barrier layer cells which embody the present invention and a graph record of the performance of one of the cells as compared with solar energy are represented in the accompanying drawings wherein:

Fig. 3 is a perspective elevational view of a cadmium sulfide barrier layer cell with a polished face radiated by the 'suns energy;

Fig. 4 is a section taken along the line 44 of Fig. 3;

Fig. 5 is a section of an N-type cadmium sulfide cell with a metal;

Fig. 6 is a section of a P-type cadmium sulfide cell with a metal;

Fig. 7 is a section of a composite cell which also embodies the present invention; and

Fig. 8 is a graph of the solar spectrum curve and a cadmium sulfide cell performance curve superimposed upon each other plotted on coordinates of wave lengths along the abscissa and current along the ordinate.

The cadmium sulfide barrier layer cell which is repre sented in Figs. 3 and 4 of the accompanying drawings is a cadmium sulfide crystal 10 with an N-type side 11 and a P-type side 12 between which sides is a barrier layer 13 across which electrons flow during the radiation of the crystal with the application to the cellof the full solar spectrum of energy from the sun 14. The crystal side which is to be radiated with energy from the sun is polished to a desired degree for optimum light penetration.

Suitable electrical contacts are applied to the crystal illustratively along each of its opposite surfaces just inwardly from the edges thereof by means suitable for the composition of the contacts, such as by electroplating, melting, painting, thermo deposition by evaporation or the like. The contacts are as. narrow as possible to expose the maximum area of the polished cadmium sulfide crystal to the action of the suns energy. Transparent contacts are within the concept of the present invention.

In selecting a contact material for application to a cadmium sulfide crystal, the relation between its work function and the work function of the cadmium sulfide crystal is taken into consideration as will be presented more explicitly hereinafter.

Contacts applied to the opposite sides of the cadmium sulfide crystal may be classified as and are referred to herein as ohmic contacts and as rectifying contacts. Most metals conduct electrical energy more efficiently in one direction than in the other. Methods of making contact to semiconductors and to ohmic or rectifying contacts are described at page 192 in the Dunlap text. The International Dictionary of Physics and Electronics, published in 1956 by D. Van Nostrand Company, Inc., Princeton, New Jersey, defines the term ohmic contact as, a contact between two materials possessing the property that the potential difference across the contact is proportional to thc'current passing through the contact. As previously stated an ohmic contact for N-type cadmium sulfide consists of a metal having a work function which is substantially the same or less than the work function of crystalline cadmium sulfide. lllustratively ohmic contact materials are the metals gallium, indium and their equivalents. A rectifying contact for N-type cadmium sulfide is a. metal which has a work function which is greater than the work function of crystalline cadmium sulfide, such illustratively as the metals copper, gold, platinum, silver and the like. Cadmium sulfide is crystallized in thin plate-like physical shape by the apparatus and the method described in patent application Number 572,170, filed March 16, 1956 by Donald C. Reynolds, the present inventor for Growth of Crystals. This application, Serial Number 572,170, describes applicants means and method for growing crystals of cadmium sulfide, zinc sulfide and the like, in a hydrogen sulfide multiple temperature, floating cup furnace. The crystals are grown of a desired degree of purity. The furnace is adapted for producing both intrinsic and extrinsic crystals. A cadmium sulfide crystal may have the characteristics of an extrinsic semiconductor imparted thereto for both P and N type crystals by introducing into the furnace cadmium sulfide crystal donor impurities or acceptor impurities. Shockley pages 12 to 15 and 237 and Kittel (1956) pages 347 and 353. Photovoltaic and barrier layer cells are described in Theory and Applications of Electron Tubes by Herbert J. Reich, published in 1944 by the McGraw Book Company, Inc. of New York City, New York, on pages 556 to 560.

The graphs 2a, 2b and 2c of Fig. 2 bear no legends on their abscissae and ordinates in harmony with the practice in the Dunlap text, such as the Fig. 7.2 to the Fig. 8.7 from page 133 to page 153 and elsewhere. The curves presented are more the expressions of theories than they are conventionally plotted lines along a series of actual, experimentally determined measured values obtained as experimental findings. The Dunlap text at page 7 refers to an extrinsic semiconductor as a semiconductor which conducts because of the presence of impurities. Kittel (1956) at page 374 includes thermal agitation and lattice defects with impurities. Intrinsic semiconductors are those which, particularly at high temperatures, conduct because both electrons and holes are thermally excited in pure material. In intrinsic semiconductors the impurity density is small as compared with the intransic carrier density at room temperature. The p-n junction is the boundary between two regions, one n-type and the other p-type. A photovoltaic cell itself produces an electric voltage when light is incident upon it. The International Dictionary states that all pure, ideal crystal semiconductors are naturally semiconducting but the property may be very small as compared with the corresponding property in impurity semiconductors and is only to be observed at high temperatures. According to the band theory of solids intrinsic semiconductors are described by the thermal excitation of electrons from the filled band the Whole width of the energy gap to the conduction band. An N- type semiconductor is an extrinsic semiconductor in which the conduction electron density exceeds the hole density, the implication being that the net ionized impurity concentration is of the donor type. A P-type semiconductor is an extrinsic semiconductor in which the hole density exceeds the conduction electron density with the implication that the net ionized impurity concentration is of the acceptor type. Introduction to Solid State Physics by Charles Kittel and published in 1956 by John Wiley & Sons, Inc., pages 270, 347 and 353 to 357, is concerned with defiicit semiconductors and impurity states.

Following the application of the contacts to the edges of the cadmium sulfide crystal photovoltaic, the resultant cell is maintained at a temperature range of illustratively about 200 C. and to 400 C. for a period of about one to five minutes for the purpose of diffusing the junction between the contact and the crystal.

A crystalline cadmium sulfide cell which is made in the described manner, consists of one contact 15 on the polished side of the crystal and a second contact 16 on the unpolished side of the crystal. Electrical energy is derived from the resultant crystal cell with its polished side under radiation from the sun by suitable means, such as by a pair of electrical energy contacting leads 17 and 18 separately attached at one of their ends to the contacts 15 and 16 and conducting electrical energy to a desired destination such as to a battery, the contacts of a galvanometer 19 or the like. A first type of crystalline cadmium sulfide cell employs the P-N junction of crystalline cadmium sulfide itself. This first type of cell consists of an N-type section of cadmium sulfide in contact with a P-type section of cadmium sulfide in the same crystal over a barrier layer 13. The term barrier layer is applied to the region in a semiconductor which is practically stripped of conduction electrons. Kittel (1956) page 388. The International Dictionary defines the barrier layer as an electrical double layer formed at the surface of contact between a metal and a semiconductor or between two metals, in order that, the Fermi levels in each material should be thesame. The term junction l l l in a semiconductor device is a region of transition be tween semiconducting regions of different electrical properties. A diffused junction is a junction formed by the diffusion of an impurity within a semiconductor crystal. A doped junction is a junction produced by the addition of an impurity to the melt during crystal growth. A grown junction is a junction produced during growth of a crystal from a melt. An n-n junction is a region of transition between two regions having different properties in n-type semiconducting material. A p-p junction corresponds inp-type semiconductor material. A p-n junction is a region of transition between pand n-type semiconducting material. Both contacts 15 and 16 of this first type of cell are ohmic contacts.

A second type of crystalline cadmium sulfide cell employs on the polished N-type side of the cadmium sulfide crystal-an ohmic contact 15, made of a metal such as indium, gallium or the like. On the unpolished side of the cadmium sulfide crystal is adhered a rectifying contact lo, made of a metal having a work function which is greater than the work function of cadmium sulfide, such illustratively as the metals copper, gold, platimum, silver and the like. h i A third type of crystalline cadmium sulfide cell of the P-type has bonded to the edge of its polished side 11 an ohmic contact 15. On the unpolished side 12 of the cadmium sulfide crystal is adhered an ohmic' contact made of a metal or an alloy having a work function, which is less than the work function of cadmium sulfide, of which illustratively are the metals indium, gallium, etc.

Modifications of the photovoltaic cell shown in Figs. 3 and 4 of the drawings are shown in Figs. 5 and 6 of the drawings. In Fig. 5 an N-type cadmium sulfide crystal 20 has an ohmic edge contact 21 of indium or gallium along the edge of one surface and its opposite surface is covered with a layer of metal 22, such as copper or silver. In Fig. 6 a P-type cadmium sulfide crystal 23 has an edge contact 24 along the edge of one surface and its opposite surface is covered with a layer of metal 25 of indium or gallium.

In rsum, the above disclosed specie of the cadmium sulfide barrier layer cell illustrated in Figs. 3 and 4 com prises the cadmium sulfide crystal 10 with one side 11 that is polished and a second side 12 that is not polished.

and between which sides is a barrier layer 13. The first type of crystal 10 consists of an N-type section at CdS contacting a P-type section of CdS over the barrier layer 13. In this first type of CdS crystal both contacts 15 and 16 are of ohmic type such as of In or Ga. In and Ga have work functions with CdS that equal or are less than the work function of CdS. A second type of crystal 10 has on its polished N-type side an ohmic contact 15 made of In or Ga and on its unpolished side 12 has a rectifying contact of Cu, Au, Pt or Ag which metals have work functions with CdS that are greater than the work function of CdS. A third type of crystal 10 is a CdS crystal of the P-type with an ohmic contact 15 of In or Ga on its polished surface 11 and an ohmic contact 16 on its unpolished surface 12. The cell shown in Fig. 5 comprises an N-type cadmium sulfide crystal 20 covered on one side with a layer 22 of copper or silver and with an ohmic contact 21 or In or Ga on the uncovered side of the N-type CdS crystal 20. The cell shown in Fig. 6 comprises a P-type cadmium sulfide crystal 23 covered on one side with a layer 25 of indium or gallium and with an edge rectifying contact 24 of silver or platinum on the uncovered side of the cadmium sulfide 23.

In Fig. 7 of the accompanying drawings is shown a composite type of cell which is within the concept of the present invention. One composite cell comprises an N-type of cadmium sulfide crystal 26 in contact with a P-type of semiconductor 27. The P-type semiconductor 27 may consist illustratively of cuprous oxide, cuprous sulfide, cupric sulfide, selenium, and the like. Contacts 28 and 2t are applied along the edges of both sides of the cell. The electrical contacts 28 and 29 are of the ohmic type with work functions the same, less than or more than the work function of cadmium sulfide depending upon whether the cadmium sulfide is N- or P-type. The elements indium and gallium are representative of contacts of the ohmic type to N-type cadmium sulfide. The elements copper and silver are representative of contacts of the ohmic type to P-type cadmium sulfide. Illustratively the contact 28 may be made of indium or of gallium and the contact 29 may be made of copper or of silver.

Another composite cell comprises a P-type of cadmium sulfide crystal in contact with an N-type of semiconductor. N-type semiconductors illustratively consist of 'germanium, silicon and the like. Ohmic contacts are used on. both sides of both types of composite cells.

i In Fig. 8 of the accompanyingdrawings is shown a spectral response curve 30 of a cadmium sulfide photovoltaic cell, such as the cellsdisclosed herein, superim: posed upon the solar spectrum curve 31. The coordinates of the curves 30 and 31 are in wave lengths along the abscissa and in current along the ordinate. The cell response curve is characterized by two peaks 32 and 33, the earlier peak 32 being at the cell absorption cutoff. At the second peak 33 each photon from the sun produces 'an electron. Electrons so produced leave the cell and are recordable on the galvanometer 19 or on other recording instruments.

The photovoltaic cells which are shown and described herein have been submitted as successfully operative embodiments of the present invention and are intended to include structural and functional equivalents and modifications thereof.

What I claim is:

1. A cadmium sulfide barrier layer cell converting solar spectrum energy directly into electrical energy comprising a cadmium sulfide crystal of ample thickness to provide an electron permeable barrier layer therewithin and having a polished first side of optimum direct exposure for energy reception for the application thereto of solar spectrum energy and having a second side, a first ohmic contact attached to the cadmium sulfide crystal polished first side and extending along the edge thereof, and a second ohmic contact attached to the cadmium sulfide crystal second side and extending along the edge thereof.

2. A cadmium sulfide barrier layer cell converting solar spectrum energy directly into electrical energy comprisihg an N-type cadmium sulfide crystal of ample thickness to provide an electron permeable barrier layer therewithin and having a polished first surface of maximum direct exposure to energy and accessible to solar spectrum energy and having a second surface, an ohmic contact attached to the polished first surface and extending along the polished first surface of the cadmium sulfide crystal, and a rectifying contact of a work function greater than the work function of cadmium sulfide adhered to the second surface of the cadmium sulfide crystal.

3. The cadmium sulfide barrier layer cell converting solar spectrum energy directly into electrical energy comprising a P-type cadmium sulfide crystal of ample thickness to provide an electron permeable barrier layer therewithin and having a polished first surface with the greatest possible area thereof exposed directly for the receipt of energy thereby and to which solar spectrum energy may be applied and having a second surface, an ohmic contact adhered to adjacent the edge only of the first surface of the cadmium sulfide crystal, and an ohmic metallic contact having a work function which is less than the work function of cadmium sulfide adhered to the second surface of the cadmium sulfide crystal and extending along the edge of the crystal second surface.

4. A barrier layer 'cell comprising P-type cadmium sulfide in contact with N-typev cadmium sulfide, and ohmic contacts attached to the, surfaces and extending along the edges of both the P-type cadmium sulfide and the N-type cadmium sulfide. v

5. The cadmium sulfide barrier layer cell converting radiation energy incident to the cell directly into electrical energy comprising a cadmium sulfide crystal with an N- type side and a P-type side between which sides is a barrier layer across which electrons flow during radiation of the crystal, an ohmic electrical contact attached to the crystal N-type side and extending along the edge thereof over a minimumgarea' to least impede the cell area of exposure to. the radiation energy incident to the cadmiumsulfide crystal, and'an ohmic electrical contact attached to the'ci'ystal P'-type side and extending along the edge thereof.

6. The cadmium sulfide barrier layer cell converting radiationenergy incident to the cell directly into electrical energy comprising an N-type cadmium sulfide'crystal with a polished side and an unpolished side. between which sides is 'a barrier layer across which electrons flow during radiation of the polished side of the crystal, an ohmic contact attached to the polished surface of the crystal and extending along the edge of the polished side of the crystal, and a rectifying contact of a work function greater than the work function of cadmium sulfide attached to the surface of the unpolished side of the crystal remote from the polished side and extending along the edge thereof.

, a 4 7. The cadmium sulfide barrier layer cell converting radiation energy incident to the cell directly into ele'c trical energy comprising aP-type cadmium sulfide crys-' tal with a polished side andan unpolished side between which sides is a barrier layer acrosswhi'ch electrons flow during radiation of the polished side of the crystal, a firs-t contact attached to the surface and extending along the edge of the surface of the polished side of the crystal and electrically conductively secured thereto, and the opposite surface of the'crystal covered with a layer of electrically conductive metal of a work function less than the work function of the cadmium sulfide crystal.

References Cited in the meet this patent UNITED STATES PATENTS OTHER REFERENCES Reynolds: The Physical Review, October 1954, vol. 96, No. 2, pages 533-534. 

1. A CADMIUM SULFIDE BARRIER LAYER CELL CONVERTING SOLAR SPECTRUM ENERGY DIRECTLY INTO ELECTRICAL ENERGY COMPRISING A CADMIUM SULFIDE CRYSTAL OF AMPLE THICKNESS TO PROVIDE AN ELECTRON PERMEABLE BARRIER LAYER THEREWITHIN AND HAVING A POLISHED FIRST SIDE OF OPTIMUM DIRECT EXPOSURE FOR ENERGY RECEPTION FOR THE APPLICATION THERETO OF SOLAR SPECTRUM ENERGY AND HAVING A SECOND SIDE, A FIRST OHMIC CONTACT ATTACHED TO THE CADMIUM SULFIDE CRYSTAL POLISHED FIRST SIDE AND EXTENDING ALONG THE EDGE THEREOF, AND A SECOND OHMIC CONTACT ATTACHED TO THE CADMIUM SULFIDE CRYSTAL SECOND SIDE AND EXTENDING ALONG THE EDGE THEREOF. 